In the production of semiconductor devices, such as integrated circuits and solar cells, various processes may be used. For example, ions of a particular species may be implanted into a workpiece to modify the electrical characteristics of that workpiece. In other embodiments, a particular species may be used to etch material on the workpiece to create features thereon. In yet other embodiments, a species may be deposited on the workpiece, for example, as a coating. One particular example of a deposition process is the addition of a layer of silicon nitride (SiNx) as an antireflective top layer for solar cells.
One mechanism to perform these various semiconductor processes is the use of a plasma processing apparatus. A plasma processing apparatus generates a plasma in a process chamber for treating a workpiece supported by a platen in the process chamber. A plasma processing apparatus may include, but not be limited to, doping systems, etching systems, and deposition systems. The plasma is generally a quasi-neutral collection of ions (usually having a positive charge) and electrons (having a negative charge). The plasma has an electric field of about 0 volts per centimeter in the bulk of the plasma. In some plasma processing apparatus, ions from the plasma are attracted towards a workpiece. In a plasma doping apparatus, ions may be attracted with sufficient energy to be implanted into the physical structure of the workpiece, e.g., a semiconductor substrate in one instance.
Turning to FIG. 1, a block diagram of one exemplary plasma processing apparatus 100 is illustrated. The plasma doping apparatus 100 includes a process chamber 102 defining an enclosed volume 103. A gas source 104 provides a primary dopant gas to the enclosed volume 103 of the process chamber 102 through the mass flow controller 106. A gas baffle 170 may be positioned in the process chamber 102 to deflect the flow of gas from the gas source 104. A pressure gauge 108 measures the pressure inside the process chamber 102. A vacuum pump 112 evacuates exhausts from the process chamber 102 through an exhaust port 110. An exhaust valve 114 controls the exhaust conductance through the exhaust port 110.
The plasma doping apparatus 100 may further includes a gas pressure controller 116 that is electrically connected to the mass flow controller 106, the pressure gauge 108, and the exhaust valve 114. The gas pressure controller 116 may be configured to maintain a desired pressure in the process chamber 102 by controlling either the exhaust conductance with the exhaust valve 114 or a process gas flow rate with the mass flow controller 106 in a feedback loop that is responsive to the pressure gauge 108.
The process chamber 102 may have a chamber top 118 that includes a first section 120 formed of a dielectric material that extends in a generally horizontal direction. The chamber top 118 also includes a second section 122 formed of a dielectric material that extends a height from the first section 120 in a generally vertical direction. The chamber top 118 further includes a lid 124 formed of an electrically and thermally conductive material that extends across the second section 122 in a horizontal direction.
The plasma doping apparatus further includes a source 101 configured to generate a plasma 140 within the process chamber 102. The source 101 may include a RF source 150 such as a power supply to supply RF power to either one or both of the planar antenna 126 and the helical antenna 146 to generate the plasma 140. The RF source 150 may be coupled to the antennas 126, 146 by an impedance matching network 152 that matches the output impedance of the RF source 150 to the impedance of the RF antennas 126, 146 in order to maximize the power transferred from the RF source 350 to the RF antennas 126, 146.
The plasma doping apparatus may also include a bias power supply 190 electrically coupled to the platen 134. The plasma doping system may further include a controller 156 and a user interface system 158. The controller 156 can be or include a general-purpose computer or network of general-purpose computers that may be programmed to perform desired input/output functions. The controller 156 may also include communication devices, data storage devices, and software. The user interface system 158 may include devices such as touch screens, keyboards, user pointing devices, displays, printers, etc. to allow a user to input commands and/or data and/or to monitor the plasma doping apparatus via the controller 156. A shield ring 194 may be disposed around the platen 134 to improve the uniformity of implanted ion distribution near the edge of the workpiece 138. One or more Faraday sensors such as Faraday cup 199 may also be positioned in the shield ring 194 to sense ion beam current.
In operation, the gas source 104 supplies a primary dopant gas containing a desired dopant for implantation into the workpiece 138. The source 101 is configured to generate the plasma 140 within the process chamber 102. The source 101 may be controlled by the controller 156. To generate the plasma 140, the RF source 150 resonates RF currents in at least one of the RF antennas 126, 146 to produce an oscillating magnetic field. The oscillating magnetic field induces RF currents into the process chamber 102. The RF currents in the process chamber 102 excite and ionize the primary dopant gas to generate the plasma 140.
To implant ions in to the workpiece, the bias power supply 190 provides a pulsed platen signal having a pulse ON and OFF periods to bias the platen 134 and hence the workpiece 138 to accelerate ions 109 from the plasma 140 towards the workpiece 138. The ions 109 may be positively charged ions and hence the pulse ON periods of the pulsed platen signal may be negative voltage pulses with respect to the process chamber 102 to attract the positively charged ions. The frequency of the pulsed platen signal and/or the duty cycle of the pulses may be selected to provide a desired dose rate. The amplitude of the pulsed platen signal may be selected to provide a desired energy.
In the case of deposition, also known as Plasma Enhanced Chemical Vapor Deposition or PECVD, the bias power supply 190 is typically not activated, allowing the ions and neutrals to drift onto the workpiece. PECVD may be used for the deposition of dielectric films and passivation films, such as but not limited to silicon oxide and silicon nitride.
In some embodiments, it is desirable to deposit material on only a portion of the surface of the workpiece. There are various methods that can be used to accomplish this. For example, one method is the use of photolithography. In this method, a photoresist material is applied to the workpiece on the areas upon which material is not to be deposited. The photoresist may be baked onto the workpiece to ensure that it remains in place. The deposition step is then performed. Afterwards, the photoresist must be removed. Often, there are several cleaning steps also required in this process.
A simpler lower cost alternative is the use of stencil masks. A stencil mask is placed atop the workpiece. The deposition process is then performed, and the material is deposited only on the areas of the workpiece that are exposed. After completion, the stencil mask is simply removed.
However, the use of stencil mask for deposition processes, especially in plasma processing chambers, also has drawbacks. For example, it has been shown that the thickness of the deposition layer, deposited using a stencil mask, is not uniform across the surface of the workpiece. Therefore, an apparatus and method that allows low cost patterned deposition, especially PECVD, is beneficial.